Eccentrically rotated rod holder for crucible-free zone melting



July 28, 1970- w KELLER 3,522,()14

ECCENTRICALLY ROTATED ROD HOLDER FOR CRUCIBLE-FREE ZONE MELTING Filed NOV. 28, 1966 3 Sheets-Sheet l LOWER HOLDER FOR SEMICONDUCTOR ROD l m47l ROTATING CYLINDER SHAFT EXTENDS ECCENTRICALLY THROUGH CYLINDER 8 ,III

\FLYWHEEL Fig. I

y 8, 1970 w. KELLER 3,522,014

ISCCENTRICALLY ROTATED ROI) HOLDER FOR CRUCIBLE-FREE ZONE MELTING Filed Nov. 28. 1966 3 Sheets-$heet 2 ECCEN CA ROTATED DER FOR y 8, 1970 w. KELLER 3,522,014

ROD 0 NE ME 0 0113 -FREE Z0 NG Filed NOV. 28, 1966 3 Sheets-Sheet S United States Patent 01 Ffice 3,522,014 ECCENTRICALLY ROTATED ROD HOLDER FOR CRUCIBLE-FREE ZONE MELTING Wolfgang Keller, Pretzfeld, Germany, assignor to Siemens Aktiengesellschaft, Munich, Germany, a corporation of Germany Filed Nov. 28, 1966, Ser. No. 597,340 Claims priority, application Germany, Nov. 30, 1965, S 100,722; June 3, 1966, S 104,126 Int. Cl. B01f 17/10 US. Cl. 23-273 Claims ABSTRACT OF THE DISCLOSURE A device for carrying out a crucible-free zone melting operation wherein a rod-shaped member of crystalline material is end-supported substantially vertically by holders in a closed chamber. The holders are displaceable laterally in axially parallel relationship to one another, and at least one of them is secured to a substantially vertical shaft. A cylinder having a substantially vertical axis and rotatable about that axis is provided, the shaft extending eccentrically through the cylinder.

My invention relates to device for crucible-free or floating-zone melting.

In a crucible-free zone-melting operation, a rod-shaped member of the material to be processed is generally gripped at both ends thereof, and is held in a substantially perpendicular position. Two holders which hold the rodshaped member at the ends thereof and which are secured to perpendicularly aligned shafts, can consequently be rotated about their axes and, in a case where the cross section of the rod-shaped member is to be varied by pulling, are displaceable relative to one another in the vertical direction. A heating device serves for producing a melting zone over a limited length of the rod-shaped member. By relative motion between the heating device and the rod-shaped member, this melting zone can be passed through the rod-shaped member along the length thereof. The zone-melting operation serves in most cases both for purifying as well as for growing monocrystals with the aid of seed crystals fused to the rod-shaped member. It has already been proposed, for the purpose of making the resistance across the cross section of the rodshaped members more uniform, to dispose the two rod portions separated by the melting zone so that their vertically extending longitudinal axes are laterally offset relative to one another. At the beginning of the zone melting process, both rod portions may be disposed along the same aligned vertical axis. One holder may then be displaced successively in a lateral direction and changed in elevation, i.e. displaced in a substantially vertical direction, relative to the second holder, and to the heating device, whereby the cross section of the crystallizing rod portion can be increased to thedesired cross section. After attaining this nominal cross section, the crystallizing rod portion and the heating device are displaced relative to one another only in a substantially vertical direction. The resistance profile across the rod cross section can be made even more uniform by displacing both rod portions laterally relative to one another also after the nominal cross section is attained.

It is an object of my invention to provide an improved device for carrying out a crucible-free or floating-zone melting operation.

With the foregoing and other objects in view, I provide, in accordance with my invention, a device for cruciblefree floating zone melting process which comprises a closed chamber in which a rod-shaped member of crystalline material, particularly semiconductor material such as 3,522,014 Patented July 28, 1970 silicon, for example, is held in a substantially vertical position by holders, pairwise gripping the ends thereof, laterally displaceable relative to one another in an axially parallel relationship. At least one of the holders is secured to a vertically disposed shaft, and is eccentrically mounted in a cylinder rotatable about a vertical axis.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in device for crucible-free zone melting, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

,The construction and method of operation of the invention, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIGS. 1 and 2 are longitudinal sectional views of two different embodiments of the device for crucible-free zone melting constructed in accordance with my invention;

FIG. 3 is a cross-sectional view of FIG. 2 taken along the lines IIIIII in the direction of the arrows during one stage of operation of the device; and

FIG. 4 is a view of the embodiment as shown in FIG. 3 at another stage in the operation thereof.

Referring now to the drawings, and first particularly to FIG. 1 thereof, there is shown a tubular member 1 having end flanges, the upper one of which, as shown in FIG. 1, being sealingly secured to a zone-melting chamber 2, also known as the receiver, which is either evacuated or filled with protective gas. The other flange is bolted to a bearing block 4 on which there are externally mounted cooling coils 14 traversed by a fluid coolant. A cylinder 3 is pro- .vided in the bearing block 4 and is rotatable about its own vertically-disposed axis. In order to effect good heat dissipation from the cylinder 3 into the bearing block 4, the bearing is in the form of a slide hearing. The cylinder 3 is rotated by a spur gear 9 afiixed thereto by Welding or the like and driven by a worm 10. A shaft 7 extends vacuum-tightly through the cylinder 3 parallel to and consequently eccentric to the rotary axis of the cylinder 3. The shaft 7 is mounted in ball bearings 8 and oil seals 6 and carries a holder 7 at the free end thereof for the semiconductor rod to be processed. The shaft 7 is driven by a motor (not shown) through an extensible elastic belt 11 and a sheave 12, and can be slidingly displaced in the axial direction thereof. At the lower end of the shaft 7, as viewed in FIG. 1, a gyratory mass such as a fly Wheel 13 of 4 kilograms, for example, is mounted so as to provide smooth and vibration-free operation of the shaft 7. The remaining components of the zone-melting apparatus, such as for example the heating device by means of which a melting zone is produced over a limited length of a semiconductor rod, are well known, for example, from Pat. No. 3,160,478 to T. Rummel et al. and from Pat. No. 3,046,379 of which the applicant of the instant application is one of the joint inventors, and are therefore not illustrated here in the interest of clarity. Reference can also be had to my copending application Ser. No. 428,933 filed Jan. 29, 196-5, now abandoned, for further details of construction of a zone-melting apparatus for use with the device of this invention.

The operation of the device for carrying out a zonemelting process is as follows:

At the beginning of the operation, substantially when a thin rod-shaped seed crystal is fused to a thicker supply rod which has been produced for example by means of deposition from silicon out of the gaseous phase thereof, the upper and the lower holders of the rod are disposed onathe .same verticalaaxis. By moving the rod holders and/or the heating devicein the axial direction of the holders, and by synchronizing their relative speeds, the melting zone produced in the rod is passed along the rod and the cross section of the recrystallizing rod portion is brought up to a desired nominal diameter. The lower rod holder 7 is simultaneously displaced laterally by actuating the worm to rotate the cylinder 3. The melting zone is passed to the edge of the resolidified rod portion due to the lateral displacement of the rod relative to the heating device. The lateral displacement of the rod should only be so large that noliquid material is able to drip from the melt. For example, with a diameter of the recrystalliz' ing rod portion of 35 mm., the lateral displacement should be approximately 10 mm.

- It has proven to be advantageous to laterally displace the lower holder 7 during increase of the rod cross section so far that after the nominal cross section it attained, no further lateral displacement is necessary, but rather, the supply rod and the growing monocrystal are then moved only in the axial direction thereof.

In accordance with a modification of the process for zone melting, the direction of movement of the supply rod and growing monocrystal is reversed after the nominal cross section is attained and after the outer limit of lateral displacement thereof is reached. By this reversal of the movement direction, the specific resistance over the cross section of the rod is further rendered more uniform. Especially good results are achieved when lateral reciprocation of the lower rod holder 7' is repeated several times during one pass of the melting zone through the rod, and particularly, if it is effected with a speed that is relatively great when compared to the speed with which the melting zone is passed along the rod. This modification of the zone-melting process is able to be carried out with the device according to my invention when the worm 10 is alternately turned clockwise and counterclockwise, for example by means of a reversible motor, with a frequency corresponding to the desired speed. The reciprocating motion of the lower holder 7 is smoother when the eccentricity of the passageway for the shaft 7 through the cylinder 3 is made equal to the desired reciprocating travel distance, and the cylinder 3 can consequently be rotated with the same rotary speed in the same direction. To achieve even smoother rotation of the cylinder 3, it is advantageous to replace the drive system of the spur gear 9 and worm 10 with a belt drive. Semiconductor rods of silicon which are produced with the aforedescribed device in accordance with such a zone-melting process only have variations of the specific resistance across the cross section of the rod that are less than 10%.

In FIG. 2 there is shown in cross section an embodiment of the device of my invention which is particularly suitable for zone melting with rod portions that are reciprocated relative to one another.

As shown in FIG. 2, in a bearing block 24 which is vacuum-tightly flanged to a zone-melting chamber 23 by means of a sealing ring 22, there is mounted a first eccentric 25 which is rotatable by means of a spur gear 30, fastened thereto as by the illustrated machine screw, that is driven by'a worm 29. An oil seal 40 ensures a vacuumtight hearing. A cylindrical second eccentric 26 is mounted in the first eccentric 25 and is driven by a motor (not shown) through an extensible and elastic belt 31 and a sheave 32 suitably secured to the eccentric 26. The bearing for the second eccentric 26 is made vacuum-tight with an oil seal 37. A shaft 27 extends vacuum-tightly through the second'eccentric '26 and a non-illustrated holder for a semiconductor rod is located at the free end of the shaft 27 within the zone-melting chamber 23. The shaft .27 .i s-driven by a non-illustrated motor through an extensible belt 33 and a sheave 34 suitably fixed to the shaft 27. Both eccentrics 25 and 26 and the shaft 27 are mounted in slide bearings. The oil seals and the bearings are protected by a plate 39 against falling particles, especially semiconductor tinsel. At the lower end of the shaft 27, a gyratory mass such as a flywheel 35 is mounted, as by means of the illustrated screws, so as to ensure a smooth and vibration-free rotation of the shaft 27. The shaft 27 has a hollow interior in which there is located a tube 36 of small diameter through which coolant is supplied thereto. The remaining components of the zone-melting apparatus, such as for example a heating device by means of which a melting zone is produced along a limited length of rod, are also known per se from the aformentioned Pats. Nos. 3,160,478. and 3,046,379 and further improved upon in my aforementioned copending application Ser. No. 428,933 filed J an. 29, 1965, now abandoned, and are not consequently further disclosed herein, as being superfluous in view of the state of the art.

The operation of the embodiment shown in FIG. 2 during a zone-melting process can be derived from FIGS. 3 and 4 which show schematically a cross section through the bearing block 24, the eccentrics 25 and 26 and the shaft 27 during two different stages of the operation of the device. At the beginning of the operation, the upper and the lower holders are again disposed in the same vertical axis which is located perpendicular to the plane of the drawing of FIGS. 3 and 4 at the intersection of the lines 41 and 42. By moving the rod holders and/ or the heating device in an axial direction, the melting zone is passed through the supply rod and the cross section of the recrystallizing rod portion is increased to the nominal diameter. The lower rod holder is simultaneously dis placed laterally by rotating the first eccentric 25 in the bearing block 24 in response to actuation of the worm 29. The melting zone is passed to the edge of the resolidified rod portion due to the lateral displacement thereof relative to the heating device and to the supply rod. The lateral displacement should be only so great that no liquid material can drip out of the melt. For example, the lateral displacement should be approximately 10 mm. for a recrystallizing rod portion having a diameter of 35 mm.

In FIG. 4 there is shown a stage of the device during the zone-melting process wherein the desired eccentricity or offset of the recrystallizing rod portion with respect to the supply rod is achieved, and from this instant the eccentric 25 is no longer rotated. After this instant, the eccentric 26 is then placed in rotation as indicated by the associated arrow whereby the lower rod portion is reciprocated laterally. In the embodiment of FIG. 2, the amplitude of this lateral reciprocatory motion is 4 mm., that is, the eccentricity varies periodically between 6 and 10 mm. By installing different eccentrics, the amplitude of the reciprocating motion can be varied.

With the described device of FIG. 2, which permits a greater freedom in selection of the amplitude of the reciprocating motion, not only is uniform specific resistance across the rod cross section attained, but also monocrystals can be produced having an even better crystal quality than are able to be produced with the embodiment of FIG. 1. It has been found that in order to achieve uniform specific resistance across the rod cross section, it is unnecessary to reciprocate the rod portion supplied to the melt and the recrystallizing rod portion between the concentric arrangement thereof and an eccentric arrangement thereof, but rather it is suf ficient to reciprocate both rod portions with lesser amplitude i.e. over a smaller reciprocatory travel distance, between two eccentric arrangements thereof. However, recrystallization of the semiconductor material is disturbed by very slight shocks or vibrations.

The devices built into the base of the receiver or zonemelting chamber can also be built into the upper surface or roof thereof so that the upper holder can be made laterally displaceable. It can furthermore also be desirable, by building in two devices constructed in accordance with my invention in one zone-melting chamber, to make both holders of the semiconductor rod laterally displaceable.

I claim:

1. In device for carrying out a crucible-free zone melting operation wherein a rod-shaped member of crystalline material is end-supported substantially vertically by holders in a closed chamber, the holders being displaceable laterally in axially parallel relationship to one another and at least one of the holders being secured to a substantially vertical shaft, the improvement therein which comprises a cylinder spaced from the holders in the axial direction of said shaft, said cylinder having a substantially vertical axis and being rotatable about said axis, said shaft extending eccentrically through said cylinder.

2. Device according to claim 1 wherein said rotatable cylinder is mounted in a bearing block.

3. In a device for carrying out a crucible-free zone melting operation wherein a rod-shaped member of crystalline material is end-supported substantially vertically by holders in a closed chamber, the holders being displaceable laterally in axially parallel relationship to one another and at least one of the holders being secured to a substantially vertical shaft, the improvement therein which comprises a cylinder having a substantially vertical axis and being rotatable about said axis, said shaft extending eccentrically through said cylinder, a bearing block, said rotatable cylinder being mounted in said bearing block and an eccentric rotatably mounted in turn in said rotatable cylinder.

4. Device according to claim 1 including a gyratory mass mounted on said shaft.

5. Device according to claim 1 including means for cooling said shaft.

6. Device according to claim 1 wherein said cylinder is mounted in slide bearings.

7. Device according to claim 1 including a bearing block in which said rotatable cylinder is mounted, and a cooling coil surrounding said bearing block.

8. In a device for carrying out a crucible-free zone melting operation wherein a rod-shaped member of crystalline material is end-supported substantially veritcally by holders in a closed chamber, the holders being displaceable laterally in axially parallel relationship to one another and at least one of the holders being secured to a substantially vertical shaft, the improvement therein which comprises a cylinder having a substantially vertical axis and being rotatable about said axis, said shaft extending eccentrically through said cylinder, sheave means secured to said cylinder, and elastic flexible belt means for driving said sheave means.

9. Device according to claim 1 wherein the crystalline material is semiconductive, and said shaft extending eccentrically through said cylinder is rotatable.

10. Device according to claim 1 wherein the crystalline material is silicon, and said shaft extending eccentrically through said cylinder is rotatable.

References Cited UNITED STATES PATENTS 2,893,847 7/ 1959 Schweickert et al.

3,113,841 12/1963 Reuschel.

3,134,700 5/ 1964 Sporrer.

3,191,924 6 /1965 Haus.

3,216,805 11/1965 Emeis.

1,999,381 4/ 1935 Wells 269 XR 2,809,905 10/ 1957 Davis.

3,228,753 1/ 1966 Larsen 23-273 NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, Assistant Examiner U.S. Cl. X.R. 279-6 

